It is known to provide ventilation systems including a heat exchanger to extract the heat from the stale inside air discharged to the exterior and transfer this heat to the incoming outside air admitted into the building. However, such systems are designed to operate in a continuous manner at a predetermined rate of flow calculated in accordance with the building size and average number of occupants. Such systems are prone to waste operating energy, since they continue to function even when the air quality inside the building is much above standard norms.
Recently, sensors for detecting the concentration of air contaminants inside buildings have been developed and made to operate the ventilation system on a discontinuous basis, namely to stop operation of the system when the air contaminants level inside the building is below an adjustable preset value.
However, such sensors, when employed in a ventilation system using a heat exchanger as above described require operation of the ventilating system in a continuous manner as, for instance in Japanese patent No. 64,332 of Mar. 5, 1988 to Matsushita Saiko Co. Ltd. and entitled "VENTILATION AIR CONDITIONING DEVICE". It has been found that when the outside temperature is below about minus five degrees centrigrade, the heat exchanger core becomes clogged with ice, thereby putting the system out of commission.
It has also been found that in various ventilation systems, it often occurs that a negative air pressure develops inside the building, that is a pressure inferior to the outside air pressure, when, for instance, exhausting of kitchen odors and the like. This causes inrushing of cold air through interstices and the like in the building structure.